Much of the transmission and distribution of electrical energy over power lines is done at some nominal frequency, typically at 50 or 60 Hz. Historically, small variations in the nominal line frequency were of little concern to electromechanical watthour metering. Electromechanical meters were limited to basic metrics such as watthours or VARhours using phase shifting transformers, and the accuracy of the results were not generally dependent on frequency.
The recent deregulation of the utility industry has created a market for products that facilitate the efficient distribution and monitoring of electrical power. For example, one reason to monitor line frequency is the increased interest in accurate measurement of harmonics on the utility's power system. Historical metering practices had only a minor concern with harmonics. Today, however, an increase in customer loads has created an increase in harmonics, and thus greater interest in harmonic analysis on power systems. This is due, in part, because greater harmonics can create even greater loading on the system. Also, the use of harmonic power instead of main power can trick an electricity meter into underestimating the amount of power that is actually consumed by a customer. Therefore, there is a greater emphasis on harmonic analysis of power systems.
In addition to increased customer demand and deregulation, the advent of electronic energy meters has allowed such analysis to be processed and displayed by the meter. For example, electronic meters are capable of determining many characteristics on the power line including: phase angles from one voltage to another voltage, phase angles from a current to a voltage, per phase power factors, per phase voltages, per phase currents, per phase voltage harmonics, per phase current harmonics, per phase and system watts, per phase and system volt-amperes, per phase and system volt-amperes reactive, and total harmonic distortion for per phase voltages and currents.
Previously, electronic meters were subject to certain limitations in the way in which power line values were sampled. For example, calculations that require the sample to be tied to a set number of line cycles were difficult to determine. This is due, in part, to the inherent and varied fluctuations that occur around a nominal frequency, like 60 Hz. More specifically, although the United States power system is said to operate at a nominal frequency of 60 Hz, in practice the actual transmitted frequency is rarely exactly 60 Hz and instead typically varies around 60 Hz. As a result, it was very difficult for any particular sample to guarantee that it had sampled a whole number of line cycles. Instead, samples typically included a portion of a subsequent and undesired line cycle that was not part of the overall calculation. Therefore, when certain calculations required one or more integer line cycles, it became necessary to compensate for the expected inability to ensure that the sample reflected a whole line cycle.
One way to compensate for the expected error included taking a greater number of samples. In this way, by increasing the “denominator” of samples, the “numerator” of extra cycle error was minimized. Although effectively minimizing the inherent error, this approach typically required very large sample times and increased memory storage.
Another approach, used in cooperation with the first approach, made certain arithmetic assumptions based on the expected nominal frequency. For example, assuming a nominal frequency of 60 hz, using a sample rate of 2370 hertz sample provides 79 individual samples over two complete line cycles. Again, the problem with this approach is that it does not contemplate variations in the nominal 60 Hz frequency, nor does it accommodate other accepted nominal frequencies, like 50 Hz. In other words, calculations like harmonics that require samples taken over integer line cycles cannot be completely compensated for after the measurements and interim calculations are done.
Therefore, it should be appreciated that there is a need for providing more accurate techniques for measuring electrical power line characteristics.